When a plurality of robots are arranged to be close to each other, the plurality of robots may at least partially share a workspace. For example, let us consider a working system in which first and second robots that respectively perform continuous first and second processes are arranged to be close to each other. In this case, the first robot is programmed to transfer a processed workpiece in the first process so as to mount it on a pallet, and the second robot is programmed to grasp the workpiece mounted on the pallet so as to perform the second process using the grasped workpiece.
In the working system set forth above, the first and second robots share at least partially a workspace on the pallet.
Such robots arranged to share a workspace may interfere with each other in the shared workspace.
An example of methods for avoiding the interference is disclosed in U.S. Pat. No. 6,212,444B1 corresponding to Japanese Unexamined Patent Publications No. H10-3308.
In the interference avoiding method disclosed in the US Patent Publication, a common area in which the workspace of a movable part of a robot and that of a movable part of a cooperative apparatus overlap and interfere with each other is set When the movable part of any one of the robot and the cooperative apparatus enters the common area, an entrance-forbidding signal is output to the other of the robot and the cooperative apparatus; this entrance-forbidding signal deactivates the movable part of the other of the robot and the cooperative apparatus.
Let us consider the interference avoiding method disclosed in the US Patent Publication is applied to a working system in which first and second robots are arranged to share a same workspace.
In this application, when an arm (movable part) of any one of the first and second robots, such as the first robot, is traveled to enter the some workspace, an entrance-forbidding signal may be output from the first robot to a controller of the second robot. In response to the entrance-forbidding signal, the controller of the second robot brakes the motion of the arm thereof.
Although the controller of the second robot starts braking the motion of the arm thereof immediately after receiving the entrance-forbidding signal, the inertia of the arm of the second robot prevents it from stopping at the moment of the start of braking, so that it has shifted by an arbitral distance since the moment of the start of braking.
The distance of the arm of the second robot from the start of braking until it is stopped, referred to as “braking distance”, hereinafter, depends on the weight of the arm and the traveling speed of the arm of the second robot.
If the controller of the second robot receives the entrance-forbidding signal when the arm thereof is located close to the shared workspace, the arm stop position may be deeply in the shared workspace.